Multiple antenna elements can be used to adapt the effective radiation pattern to varying channel and interference conditions. In its simplest form the signal is transmitted from all the antennas with antenna specific complex weights, i.e. a weight that is determined by an amplitude value and a phase. By this means it is possible to achieve a directivity gain when focussing the transmitted energy to the intended receivers instead or merely spreading energy uniformly over the whole coverage area.
There are several techniques commonly used in second and third generation systems: Conventional beamforming assumes a reasonably well-behaved propagation and requires a calibrated antenna array so that a direction of arrival can be determined from the average correlations of the received signals. Based on the direction and knowledge of the array geometry, transmit weights are determined as functions of the direction. In fixed multibeam antenna systems a set of fixed beams with different pointing directions is used and subset of the beams are selected for downlink transmission. Typically, the beam in which the most power is received is used for transmission. This technique has less requirements on calibration compared to conventional beamforming but still requires a reasonably well-behaved propagation environment. Yet another technique is closed loop transmit diversity where the transmitter transmits antenna specific pilots from which the receivers can estimate the channels. The transmitters and the receivers use an agreed set of weights. The receiver determines the weight in the set that best matches the channel and signals this to the transmitter. The weights are in essence adapted to the instantaneous downlink channel.
Another possibility is to transmit antenna specific training signals from which the terminals can estimate the complex valued downlink channel responses. The estimated downlink channels are then fed back to the receiver and used to construct transmit weights, see e.g. Gerlach, Paulraj: “Adaptive Transmitting Antenna Arrays with Feedback”, IEEE Signal Processing Letters, Vol 1, No 10, p 150-152, October 1994. Such an approach considers only the channel, and a practical implementation will require some form of quantization and agreed code book in order to fed back complex valued channel responses.
A simpler approach is considered in Banister, Zeidler: “A Simple Gradient Sign Algorithm for Transmit Antenna Weight Adaptation with Feedback”, IEEE Transactions on Signal Processing, volume 51, No. 5, p 1156-1171, May 2003. Here, a single weight vector for a single user is tracked. This is done by creating two weight vectors as slightly randomly perturbed versions of the currently tracked weight vector. Multiplexed pilot signals are then transmitted so that the receiver can measure the received power on the two weight vectors and report back to the transmitter which pilot was received with the highest power. The transmitter will then replace the currently used weight vector with the weight vector corresponding to the strongest received pilot signal. The process is then repeated in a continuous fashion. Thus, only a single user is considered and only a single bit is fed back to the transmitter for each measurement period. A similar method is proposed in Banister, Zeidler, “Feedback Assisted Transmission Subspace Tracking for MIMO Systems”, IEEE Journal on Selected Areas in Communications, Vol. 21, No. 3, p 452-463, April 2003. In this case antenna specific pilots are used in addition to the pilots used to transmit data, and only a single feedback is generated.